This paper examines the influences of the foundation flexibility and the initial irregularities on the seismic response of cylindrical tanks. A calculation method for describing those behaviors is showed and dynamic shaking table tests are performed. The results of dynamic tests agreed fairly well with the results obtained by numerical calculations. It is concluded that the base fixty conditions will significantly change the seismic response of tanks. It is also concluded that significant high radial accelerations are induced on account of the irregularities, however, stresses induced by the irregularities are less significant.